There are numerous examples known from the patent literature of the preparation of electrodeposition primers. Compounds of tin- and/or of bismuth, more particularly, are employed as crosslinking catalysts in these materials. In recent times the bismuth compounds have been used preferentially as crosslinking catalysts, since in addition to their high activity they also possess a low toxicity as compared with the tin compounds. The use of bismuth compounds as catalysts in the formation of urethane structures from isocyanate groups and hydroxyl groups is already well established (J. H. Saunders and K. C. Frisch, Polyurethanes, Chemistry and Technology, from High Polymers, Vol. XVI, Part 1, Interscience Publishers, a division of John Wiley and Sons, New York, 4th Printing, July 1967, page 167). To date, however, the use of bismuth compounds as catalysts in connection with the preparation of electrodeposition primers has been very limited. In EP 0 642 558 the bismuth compounds that are candidates for the use of electrodeposition primers are significantly restricted, since the readily available salts of relatively long-chain acids cause disruptions as a result of oily exudations when used in cationic binders. Furthermore, as a result of being mixed into the binder or into a pigment paste, inorganic bismuth compounds are said to be difficult to disperse and in this form to have only a low catalytic activity. German patent application DE 102 36 350 A1 describes electrodeposition primers comprising bismuth subsalicylate which exhibit good leveling, are free from surface defects, and ensure effective corrosion control. These known electrodeposition primers, however, require a relatively high baking temperature in order to achieve sufficient crosslinking. In the as yet unpublished patent application DE 10 2007 038 824 A1, electrodeposition primers are described that comprise basic bismuth nitrate. As a result of this it is possible for the crosslinking reactions to take place at lower baking temperatures.
European application EP 0 433 783 A1 describes aqueous electrocoats which comprise dispersed polymer particles. These polymer particles include urea-aldehyde resins, which form crosslinked, infusible masses. WO 98/33835 discloses cathodically depositable electrodeposition primers which comprise amino resin crosslinking agents such as alkylated urea-formaldehyde resins. The electrodeposition primers claimed in WO 87/02717 may likewise comprise crosslinking agents such as alkylated urea-formaldehyde resins.
The films of the electrodeposition primers known in the prior art, however, do not have uniform, homogeneous surfaces and do not possess properties of elasticity at low baking temperatures.
It is an object of the present invention to provide a new electrodeposition primer with which the crosslinking reactions ought to take place at very low baking temperatures. At the same time the film of the electrodeposition primer ought to have a uniform and homogeneous surface, even in critical edge regions. Moreover, the film ought to possess properties of elasticity.